Cedar shingles directly over rigid
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| Posted in Energy Efficiency and Durability on
Has anyone experienced any issues w/ installing cedar shingles directly over 1″ rigid and nailed back to 1/2″ sheathing behind the rigid? Any movement to the shingles? Is it necessary to install a building wrap or felt b/w the shingles and rigid assuming the rigid has taped seams?
Dow recommends wood sheathing or furring strips over the rigid, but not sure if this is necessary with 1″ of foam.
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I have no direct experience with such an envelope because I would never consider building one like that. Cedar shingles are a wonderful cladding because they are so breatheable, but they need a breatheable membrane or sheathing behind them. The small-diameter fasteners required for cedar shingles would tend to be unstable if not embedded directly into wood sheathing (plywood or strapping, if not sawn boards).
#15 felt is the appropriate underlayment for cedar shingles, which swell and shrink with changes in ambient relative humidity and rainfall. If they are finished on the outside, it becomes more important for them to be able to breathe to the inside. I would never install cedar shingles on plastic, either foam board or polymeric housewrap (with the possible exception of Cedar Breather).
Chris,
Don't do it. Builders experimented with wood siding over foam in the 1980s, with disastrous results, including cupping and splitting.
Here's a link to a JLC article from June 1986 on the problem:
http://www.jlconline.com/cgi-bin/jlconline.storefront/4bc6672700b59adc27190a32100a0667/Product/View/8606wood
Martin,
You would suggest (other than double wall, Larsen truss, etc.) to cover rigid with another layer of wood sheathing or strapping? Just looking for a simplified method for achieving a thermal break w/ typical stud construction.
Chris,
On my own house, built in 1980, I used horizontal 1x3 strapping (rough-sawn green lumber), 5 inches on center, over the foam sheathing -- then cedar shingles. Worked fine.
You might want to consider an 'ISO Sandwich'... this example is from Europe but you can find it made in the US.
http://www.termoisolanti.com/pages/en/products-menu/thermal-insulators/structural-panel-thermal-insulators/isosandwich.php
Martin,
Doesn't the horizontal strapping interrupt the drainage plain? Wouldn't it be better to install the horizontal strapping over vertical furring strips? Maybe this is overkill...
Chris,
Rather than 2 layers of sheathing on either side of the foam, you could do let in wall bracing for structure, the 1" foam and then the sheathing. I'm not an engineer but I think it would be stable and would save some money over 2 layers of sheathing.
I chose to go double studs with dense pack cellulose so I could use shingles. After reading an article in Coastal Contractor magazine, I used the 3/8" thick DCI Cedar Vent and Rafter Vent products to create a vent space. I ripped them in half lengthwise and put them up on 5" centers.
Brett, there's no better drainage plane or breatheable cladding than cedar shingles.
Donald, let-in T-bracing combined with screwed drywall on the interior of exterior walls combines to create as much shear bracing as most houses will ever need (except in hurricane or seismic zones).
Brett,
In theory, horizontal strapping interferes with water drainage. In reality, though:
1. If you have a significant flow of liquid water down the outside surface of your foam sheathing, you have a major flashing disaster on your hands. Something is wrong. Really, people misunderstand the purpose of a rainscreen gap. It isn't so much to provide a path to liquid water; it's there to provide a capillary break and as an aid to drying.
2. Cedar shingles are extremely permeable and dry very rapidly. Any moisture that finds its way behind the siding dries in a flash.
3. When I've had opportunities to open up my walls and talk a look, everything is hunky-dory and peachy-keen in there.
Martin,
What would you use to fasten the strapping through the rigid? We live on the coast, so typically would use stainless for shingles. Should we use stainless screws for the strapping just as insurance.
Also, I'm assuming the rigid (if taped properly) acts as the WRB and no other paper (felt of building wrap) is necessary over the rigid.
Finally, I'm using clad windows with a nailing fin. Does it matter if I install the windows against the rigid or pad out around the opening the thickness of the strapping and install to that? I am thinking you could get a better flashing detail against the rigid, but then the casing will be out in front of the face of the clad frame (maybe not a bad thing, could look more like a wood frame window).
Donald,
We are in a 110mph zone, so unfortunately wood sheathing is the way to go for lateral bracing. Almost all of the wall area without windows is utilized as a shear wall.
Chris,
Save your money. You don't need stainless screws to fasten your strapping. Ordinary coated steel or galvanized screws are fine.
There are two ways to detail your WRB. It's possible to use the foam as a WRB, using tapes or flexible flashing to ensure watertightness at horizontal seams and window heads. (Alternatively, these horizontal seams can be flashed with metal Z-flashing or even 6-mil poly. This sometimes requires the flashing to be carried up to the next highest horizontal seam.)
I prefer to see plastic housewrap used as a WRB, on top of the foam. That makes it easier to integrate the WRB with your window flanges.
I would install "picture frames" around your rough openings, using lumber of the same thickness as your foam. That gives you something secure to nail or screw your window flanges to.
I'm not sure why your window trim ends up in the wrong plane. Here in New England, exterior side casing and head casing are always proud of the siding, and the thickness of the trim under the window sill, if any, is chosen so that the window sill is proud of the under-sill trim.
I meant that the side and head casing would be proud of the face of the clad window frame. Typically w/ a clad frame unit (Marvin Aluminum, Andersen Vinyl) the "face" of the window frame is flush or just proud of the casing. Not a big deal in either case....
Any particular rigid product you would recommend for this application? I was thinking of using 1" foil faced Dow thermax, which I think is R6.5. I'll still plan on taping the seams of the rigid and then covering w/ a house wrap w/ taped seams as well.
I've seen at the base of the wall people using insect screen and/or some kind of flashing kickout. Is that only necessary with thicker (2" or greater) rigid installations?
It seems to me that you're either forcing the rigid into the assembly or forcing the shingles. Shingles are awesome if they can breathe and be fasteneted to a rigid surface (perhaps over some homeslicker) If you want rigid on the outside, I think you want a something that will span vertical strapping members like cedar clapboards, or maybe Nichiha premium shakes (they come in 8' lengths. but weaving corners would be tough.)
Martin,
Are there any code implications with the 1" ext. rigid (r-6.5)? Someone mentioned to me that a 2x6 wall without a warm-side vapor retarder must have at least R-7.5 on the exterior. Are they implying that because the rigid is on the exterior the wall must be able to dry to the interior and as a result you do not want an interior vapor retarder. I have not been able to track it down in the code (MA State Building code, largely based on IRC? I think).
Chris,
To determine requirements of a local building code — for example, the Massachusetts State Building Code — there is no substitute for looking it up. You need to either buy a copy of the code book or find out if it is posted online.
According to Table N1102.5.1 in the 2007 Supplement to the IRC, homes in Climate Zone 5 (including Massachusetts) with foam sheathing need a minimum of R-5 foam for 2x4 walls or R-7.5 foam for 2x6 walls. This is prudent advice, whether or not the Massachusetts code includes a similar provision.
So I think I have finally nailed down the system
2x6 @ 24" o.c.
dense pack cellulose
5/16" advantech sheathing
1 1/2" foil faced rigid
1x strapping @ 5" o.c. or another layer of 1/2" sheathing (waiting on the GC's take)
plastic wrb taped seams
white cedar shingles, 5" exposure
Thanks for all of the responses. It has been very helpful.
One final question...what type of screening should be used at the base of the wall to prevent insect infiltration into the rigid?
Thanks
If you use a secondary WRB (which is unnecessary over foil-taped, foil-faced iso), it must be behind the furring. That is both so that it will not deflect (and open seams) under wind pressure and to keep it away from the surfactants (tannins) in the cedar.
Robert,
Is taping the seams of the foil face sufficient for creating the drainage plane or do you use "z" flashing at horizontal and vertical seams?
I thought by using the WRB over the rigid, I could get a better drainage plane with less work than flashing all of the seams of the rigid.
It sounds like from your comment you are not a fan of plastic wraps directly behind cedar? Generally, we tend to allow use felt only behind cedar, but I had been reading that the newer generation plastic wraps perform much better behind cedar than they used to.
Yes, you are correct, if we go with strapping the wrb would be behind the strapping...
Any comments on the base insect screen detail?
Thanks
Chris,
If I used exterior foam board (which I don't), I would certainly use z-flashing since tape is not a reliable long-term barrier (though the foil tape sticks to foil facing far more tenaciously than other tapes do to housewrap. The additional difficulties in integrating the WRB and flashings with exterior foam is one of many reasons I don't use it.
If I had to use a plastic WRB directly behind cedar (which I wouldn't), it would probably be Hoem Slicker plus Typar. I don't use rainscreens, both because of the additional challenge in integrating flashings and because it creates a fire channel. But if I did, I would use fiberglass window screening (some offer it in narrow rolls for this purpose). This is to keep flying insects from nesting in the rainscreen cavity, but does nothing to prevent boring insects from getting into the foam board - that requires a metal insect flashing.
Update - In talking to the contractor, we have decided to go with a second layer of plywood over the rigid rather than horizontal battens. It simplifies a lot of the casing details and labor cost (althought that is balanced out in material cost).
My question is:
Where should the WRB go in this case? I 'm thinking it should be on top of the second layer of plywood directly behind the shingles. Treat the second layer of plywood like you would the primary sheathing layer on a standard framed house (sheetrock, studs, sheathing, felt, siding).
The windows will be installed on the second layer of sheathing, so it seems to work well to treat that layer as the drainage plane. We will still tape the rigid joints.
The WRB should be on the weather side, directly behind the cladding, and it should be #15 felt so it won't be degraded by the surfactants in the cedar, ASTM D226 felt if you can find it rather than the more common (and thinner) ASTM D4869.
Update #2 - Has anyone tried using typar w/ homeslicker over the rigid and attaching the shingles directly over the typar, fastened through the rigid back to the sheathing w/ 2.5" ring shank nails?
JLC had an article with this application a few months back except they were only using 1", which has been noted in this discussion as not appropriate for this climate. They list the upcharge as being 9-10k (not clear if that only accounts for rigid, sealing rigid and building paper or if it also includes cost for new window trim). The estimate seems pretty high if is only includes material and labor directly related to the rigid and wrb.
Any thoughts...
Additional info...and in the proposed situation above, we would not use a foil faced product.
With new Certainteed 5" sawmill plastic shingles, would there be an issue nailing these directly through 1 1/2" of foam? Since they are plastic, drying/rotting would not be an issue. Any thoughts?
Use the recommended thickness of foam for your climate. Using less increases the danger of mold in your walls.
When installing windows over foam, install something the same thickness as your strapping over the foam - not in place of the foam. We typically use Advantech around openings where we've used exterior foam. This puts the windows on the correct plane in relation to the siding.
Using another layer of plywood or OSB over the foam and under the shingles guarantee that the shingles (if wood) will not dry properly on the back, increasing the danger of the shingles cupping and shortening their life. Those are the issues that the strapping addresses. Sheathing with a layer of felt underlayment ("tar paper") would help.
I've taken apart walls where siding was nailed through foam - after 30 years the foam was in pieces, the holes enlarged and the siding was loose. Was and is a bad idea.
Steve,
The first step in this type of inquiry is to check the manufacturer's installation instructions. In the case of the product you are talking about -- CertainTeed Cedar Impressions Individual 5" Sawmill Shingles -- the installation instructions are (like those of most building products) posted online.
Those instructions state: "The shingles must be installed over a solid flat substrate with nail holding strength such as plywood or OSB (minimum 7/16" thick)."
-- Martin Holladay